strange artifact when rendering volume
The artifact is probably due to the Color Ramp transition method Ease and the fact that your ramp control points are really close to each other. Even though this is inside the planet, it impacts your final render because of the 2 step compositing. I suggest trying B-Spline which should produce smoother transition.



An alternative is to use RGB Curve and play with the curve shape which may give you more control than the ramp for this purpose. Note the Vector Handle vs Auto Handle which impact the shape of the control point, respectively sharp and smooth.